Updates to x/net/http2 git rev c24de9d5

Change-Id: I3d929ae38dca1a93e9a262d4eaaafee1d36fa839
Reviewed-on: https://go-review.googlesource.com/17896Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>